1. Field of the Invention
The present invention relates to electronic circuitry of the type used in an integrated circuit for detecting when the supply voltage to the integrated circuit falls below a predetermined level indicative of a possible interruption in the functioning of the integrated circuit or corruption of data within RAM memory coupled to the same supply voltage.
2. Prior Art
Voltage threshold detectors and low voltage detector circuits are well known in the prior art. Typically these circuits generate a logic signal output indicating whether a monitored voltage is either over or under a specified reference level. This output signal may then be used to disable other circuits in order to prevent improper operation caused by low voltage conditions.
In applications such as automotive electronic systems it is important to maintain adequate supply voltages to integrated circuits controlling such key functions as engines, chassis/suspension, anti-lock brakes, etc. For example, in an engine control computer it is highly desirable to maintain current indications of engine performance in a look-up table contained in RAM memory. These current values allow the engine control computer to provide optimum performance of the engine depending upon various conditions such as changing altitude, temperatures, humidity, air mass/density, etc. These values are typically updated on a frequent basis while the vehicle is operating. Critical performance parameters, such as fuel economy, can be degraded if the data in the RAM memory is corrupted and the engine control computer thereafter uses the incorrect data. Under such circumstances it is preferable to use preset values optimized for engine startup conditions, and then redevelop the data required to optimize performance based upon actually encountered conditions.
One major concern in low voltage detector circuits found in the prior art is temperature compensation. For example, automotive electronic modules and systems must perform to demanding electrical specifications over a wide range of temperatures from sub-artic cold through desert heat without appreciably affecting the performance of the vehicle systems. Another major concern is the wide range of voltages which may appear from the vehicle's electrical power system due to transient loads caused by starting major electrical systems such as the starter motor, the air conditioner, headlight or other lighting systems, etc.
Given the wide range of ambient temperatures and supply voltage variations encountered in typical automotive environments, it therefore becomes important to monitor and maintain the proper performance of key automotive electronic systems. Since supplemental voltage reference sources are not available in typical automotive applications, it is desirable to utilize a low voltage detection circuit that not only monitors the voltage from the vehicle power supply source, but also is powered from the same voltage source. This complicates the design of such low voltage detector circuits because without the use of external voltage references the low voltage detector circuit may provide false indications of vehicle power supply conditions when the power supply voltage drops to very low levels.
Several different attempts to design low voltage detection circuits that are functional over wide temperature variations and supply voltage conditions are illustrated in the prior art. For sake of example, Pommer, in U.S. Pat. No. 4,613,768, discloses a temperature dependent voltage reference and voltage comparator comprising a three terminal circuit. While this circuitry is capable of providing adequate voltage reference comparisons over a wide range of temperature variations, the circuit is subject to providing false indications of supply voltage conditions as the supply voltage itself drops below a minimum critical value.
Waldhauer, in U.S. Pat. No. 4,882,761, discloses a programmable low voltage audio compressor circuit suitable for use with hearing aids. The device includes a bandgap reference cell which is used in conjunction with a preregulator and operational amplifier to control the supply voltage more accurately as the battery powering the hearing aid deteriorates in performance. As with other prior art references, this invention does not appear to resolve the problem of providing an accurate indication of power supply status in low voltage and low temperature conditions.
Johnson, in U.S. Pat. No. 4,324,216, discloses an electronic ignition control system for an internal combustion engine, including a sophisticated electronic timing advance system. While the inventor recognizes the wide temperature and voltage fluctuations expected for proper operation of the system in the automotive environment, the low voltage detection circuits apparently do not provide for sensing and then disabling critical electronic functions in response to the power supply voltage dropping below the predetermined critical level.
While the prior art may reflect the use of a differential comparator circuit in combination with a bandgap type input, the comparator in the prior art circuits would cease to function when the supply voltage V.sub.DD drops to very low levels. This failure mode results in the prior art circuitry going into an undetermined state when the voltage drops below that required for the proper operation of the combination of the differential comparator and bandgap input circuitry.
In contrast to the prior art, the present low voltage detector (LVD) circuit provides a normal output signal when the power supply voltage is above a specified minimum value and a second or failure signal when the power supply voltage, also known as V.sub.DD, falls below this predetermined limit. This specified minimum value of V.sub.DD, also known as the "trip point", is usually relatively low (e.g., less than two volts) and controlled within a narrow tolerance over the entire temperature range of operation (e.g., -40.degree. to +125.degree. centigrade). Typical specifications for automotive applications of this type require that the low voltage detection circuit meet or exceed a 1.8 volt +/-0.3 volt trip point specification, and that the circuit can be economically fabricated on a conventional n-well CMOS digital semiconductor manufacturing process.